A thermosensitive recording process is a recording process using heat generated by a thermosensitive recording head (hereinafter, is referred to simply as a head) in accordance with input signals, this causes a fusion contact between a color developer and a color former on an image receiving sheet in contact with the head, whereby color images are obtained. The speed of the thermosensitive recording process is dependent on the quantity of information capable of being transmitted using a telephone circuit. Moreover, this process provides a primary coloring system without need of development and fixing steps, and since the wear and tear of the head are very less, the process has been rapidly spreading to applications in information processing equipment such as printers, facsimile machines, etc.
Moreover, the development of various kinds of office devices and variety of uses for these devices have rapidly progressed and hence the development of a thermosensitive recording material capable of meeting each requirement has been needed. For example, as a thermosensitive recording material capable of meeting the increase of the speed of a recording device, a thermosensitive recording material capable of giving clear images having a high density even using only a small amount of printing energy has been required. To meet the demand, it has been necessary to investigate not only the thermosensitive recording layer but also the support, and use of synthetic resin films as the support in place of conventional ordinary papers has increased.
For example, as a thermosensitive recording material using a resin film containing an inorganic fine powder, a thermosensitive recording material using a biaxially stretched resin film layer having fine voids, the content of the fine voids being from 40 to 100 cc/100 g, as one element of the support for a thermosensitive recording layer and a thermosensitive recording material wherein on this type of biaxially stretched resin film layer is further laminated a film layer having the same material as the foregoing resin film or a different material therefrom are disclosed in U.S. Pat. 4,996,182 and JP-A-2-70479 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
However, in using the resin film layer meeting such voids, a support for thermosensitive recording, which has excellent resolving power, provides clear images having a high density, and where curling due to heat even after printing does not occur can not be obtained. Accordingly, for obtaining clear images having a high density, it is necessary to improve the smoothness of the surface, which is property other than voids.
Thus, reducing the compounding amount of the inorganic fine powder for improving the surface smoothness of the support has been attempted but in this case, the amount of voids in the film formed by stretching is decreased to reduce the cushioning property and lower the density of images formed. Therefore, the foregoing problem could not be solved in the manner tried.
Also, with the recent marked increase in the amount of information, high-speed recording devices (requiring about 10 seconds of a recording time for an A4-size page (210 mm.times.297 mm) has been developed in place of the earlier so-called low-speed recording devices (requiring about 3 minutes of the recording time for an A4-size sheet), and further ultrahigh-speed devices have been investigated. With the tendency toward an increase in the recording speed, it was attempted to increase the compounding amount of an inorganic fine powder in an inorganic fine power-containing resin film for roughening the back surface to lubricate the back surface. However, the amount of voids of the film formed by stretching in this approach is increased and the surface smoothness is reduced. This results in lowering the density of the images formed. Thus clear images having an excellent resolving power and a high density can not be obtained at high speed.